In many industries, like the petrochemical and chemical industries the processes employ reactors in which chemical reactions are effected in the components of one or more reaction fluids under given temperature and pressure conditions. Many of these reactions generate or absorb heat, to various extents, and are, therefore, exothermic or endothermic. The heating or chilling effects associated with exothermic or endothermic reactions can positively or negatively affect the operation of the reaction zone. The negative effects can include among other things: poor product production, deactivation of the catalyst, production of unwanted by-products and, in extreme cases, damage to the reaction vessel and associated piping. More typically, the undesired effects associated with temperature changes will reduce the selectivity or yield of products from the reaction zone.
One solution for controlling the changes in temperature associated with the heats of various reactions has been to operate several adiabatic reaction zones with intermediate heating or cooling between the different reaction zones. In each adiabatic reaction stage, all of the heat liberated or absorbed during the reaction is transmitted directly to the reactive fluid and the reactor internals. The degree of heat release and the tolerance for temperature change determines the total number of adiabatic reactor zones required in such arrangements. Each zone or adiabatic stage of reaction adds significantly to the overall cost of such a process due to the equipment expense of adding piping and heaters or coolers for intermediate stages of heat transfer to a reactant that passes through the reaction zones. Therefore the number of adiabatic steps is limited and such systems offer at best a stepwise approach to isothermal or other controlled temperature conditions. Moreover, the breaking up of a reaction zone into a series of reactors with intermediate heating or cooling of reactants, especially interferes with reactor arrangements that have continual addition and withdrawal of catalyst from the reaction zone.
Other solutions to the problem of temperature control under the influence of different heats of reaction have employed direct or indirect heating or cooling within the reaction zone. Direct heating or cooling utilizes a compensating reaction having a directionally different heat requirement that occurs simultaneously with the principal reaction. The counter balancing reaction offsets heat release or heat adsorption from the principal reaction. One of the simplest forms of such an arrangement is an endothermic process that uses oxidation of hydrogen to heat reactants in an endothermic reaction.
Another solution has been the indirect heating of reactants and/or catalysts within a reaction zone with a heating or cooling medium. The most well known catalytic reactors of this type are tubular arrangements that have fixed or moving bed catalysts. The geometry of tubular reactors poses layout constraints that require large reactors or limit throughput.
Indirect heat exchange has also been accomplished using thin plates to define channels that alternately retain catalyst and reactants between a heat transfer fluid for indirectly heating or cooling the reactants and catalysts. Heat exchange plates in these indirect heat exchange reactors can be flat or curved and may have surface variations such as corrugations to increase heat transfer between the heat transfer fluids and the reactants and catalysts. Although the thin heat transfer plates can, to some extent, compensate for the changes in temperature induced by the heat of reaction, the indirect heat transfer arrangements are not able to offer the complete temperature control that would benefit many processes by maintaining a desired temperature profile through a reaction zone.
Many hydrocarbon conversion processes will operate more advantageously by maintaining a temperature profile that differs from that created by the heat of reaction. In many reactions, the most beneficial temperature profile will be obtained by substantially isothermal conditions. In some cases, a temperature profile directionally opposite to the temperature changes associated with the heat of reaction will provide the most beneficial conditions. An example of such a case is in dehydrogenation reactions wherein the selectivity and conversion of the endothermic process is improved by having a rising temperature profile, or reverse temperature gradient through the reaction zone.
A reverse temperature gradient for the purposes of this specification refers to a condition where the change in temperature through a reaction zone is opposite to that driven by the heat input from the reaction. In an endothermic reaction, a reverse temperature gradient would mean that the average temperature of the reactants towards the outlet end of the reaction zone have a higher value than the average temperature of the reactants at the inlet end of the reaction zone. In an opposite manner, a reverse temperature gradient in an exothermic reaction refers to a condition wherein reactants towards the inlet end of the reactor have a higher average temperature than the reactants as they pass toward the outlet end of the reaction section.
It is known to vary the heat transfer coefficient down the length of a heat transfer surface when indirect heat exchange is used to control the temperature profile along the length of a reaction zone. Changing the heat transfer coefficient in this manner compensates for the loss of temperature of the heat exchange medium as it passes along the length channels that confine the reactants.
In addition to changes in the temperature of the heating medium as the reaction progresses, the heating or cooling requirements for the reactants also change as the reaction progress. For example, in the case of many catalytic processes the reactants undergo an initial period of high activity followed by a rapid fall off of activity as the reaction goes to completion. An endothermic process such as reforming may have a high initial loss of heat followed by a gradual drop of heat consumption as the reaction ceases. Where the reactants and heat exchange medium flow countercurrently it is difficult to simultaneously compensate for both the change in heating medium temperature and heat demand. Passing the reactant stream through the reactant channels in a direction transverse to the principal flow direction of the heating medium causes too much or too little heating to occur at one end of the reaction zone as the heating requirements change along the length of the reaction zone. This can again prevent a process from operating with the most beneficial temperature profile for the ongoing reaction.
The achievement of a desired temperature profile encounter further difficulty when the reaction section uses a fixed bed of catalyst. With a fixed bed of catalyst deactivation and reaction kinetics combine to make the achievement of isothermal or other controlled temperature profiles a dynamic operation. In many chemical reactions, the highest amount of activity occurs as the reactants initially contact the catalyst in the beginning of the bed. This is true while the bed of catalyst is new and has not experienced deactivation. For example, in an endothermic process, operating with a fixed bed of catalyst, a high degree of activity requires high heat input and can result in a decrease in temperatures. At the same time, the downstream portions of the catalyst bed that receive a mixture of product and a small amount of unreacted components has much lower heats of reaction and temperature can rise relative to the inlet portion of the bed. As time progresses and catalyst deactivation occurs, the relative requirements for heat input through the bed reverse. The less active catalyst now located at the upstream portion of the bed requires less heating due to the reduced conversion of the reactants as compared to the downstream portion of the bed where a majority of the reactants are now converted.
It is an object of this invention to provide a reactor that offers greater temperature control of reactants by the indirect heating or cooling of a reaction stream by a heat exchange fluid within a reaction zone.
It is a further object of this invention to provide a process and apparatus used for indirect heat exchange of a reactant stream with a heat exchange stream for controlling the temperature profile through the reaction zone.
Another object of this invention is to provide a process that uses indirect heat exchange with a heat exchange fluid to maintain substantially isothermal conditions or a reverse temperature gradient through a reactor.